It is known to provide frequency compensation circuits. An example of this is found in Solomon, "The Monolithic Op Amp: A Tutorial Study," IEEE J. Solid-State Circuits, December 1974. In the Solomon article, a capacitor is included in the circuit between stages of an op-amp for performing frequency compensation. This capacitor "splits" poles of the circuit so that one pole is dominated by the compensation capacitor and the other pole is dominated by a load capacitor. By splitting the poles, the higher frequency pole has greater than a unity gain frequency. This pole splitting technique maintains circuit stability by preventing the phase shift becoming greater than 180.degree. at frequencies less than the unity gain frequency.
Low drop-out regulators, i.e., regulators with a small difference between the input voltage and the regulated output voltage, and other circuits that drive a load to a voltage near one or both supply rails, can be difficult to compensate. Such circuits often have a large load capacitor in parallel to a load resistor. If the load capacitor is known and dependable, it can be used for part or all of the frequency compensation for the circuit. Generally, however, this capacitor is not dependable because it was not particularly selected to match the particular components of the low drop-out regulator at issue.
A frequent problem when large capacitors are included in a circuit particularly is the effect of the equivalent series resistance (ESR). For example, electrolytic capacitors can have an ESR ranging from many hundredths to several ohms. Even more difficult to deal with is that the ESR can increase over time. While the ESR may not interfere with filtering, it does introduce into the frequency response a zero that can stop the roll-off of the gain and can extend the bandwidth to higher frequencies at which other poles can affect the frequency response. Another consideration is that gain and loop stability are further complicated by the wide variability of resistive loads.
The problems generated by the variation in possible resistive and capacitive loads are most acute in a regulator with a high output impedance element, such as a collector or drain. Load capacitance may be addressed by indicating to users and potential users, through a product specification, that a minimum capacitance between the output terminal and ground that is required, and that this capacitor must have an ESR in a particular range. This approach, however, relies on users for proper selection of the load capacitor.
In one known regulator circuit, an operational transconductance amplifier ("OTA") receives a feedback voltage derived from a regulator output voltage at its inverting input via a voltage divider. A reference voltage connects to its non-inverting input. The OTA compares these voltages and provides an output current to a load to equalize the feedback and reference voltages. A load can include a load resistor, a load capacitor C.sub.L with its inherent ESR, and even an additional current source which appears as a high impedance load.
To regulate the load voltage, the transconductance (g.sub.m) of the OTA is large so that the OTA will provide the necessary load current if there is a small voltage difference at the inputs. Because an OTA will have internal poles, the unity gain frequency should be located well below the frequencies of these poles. This limitation requires any load capacitor C.sub.L to be relatively large. This is usually not a problem because there typically is a desire to make C.sub.L large enough to filter effectively against the lead resistance. This remains true as long as the ESR of the load capacitor is small enough.
Load capacitor C.sub.L causes a pole at very low frequency and the gain decreases until the reactance of C.sub.L equals the ESR. At this point, there is a zero of response, and the gain stops decreasing with increased frequency. If the ESR is greater than the reciprocal of the product of g.sub.m and an attenuation factor from the voltage divider, this zero response occurs at a frequency below the desired crossover frequency. At higher frequencies, therefore, nuisance poles of the OTA can destabilize the feedback loop.
Another approach to control regulation of the load voltage is to cascade two OTAs and provide a compensation capacitor that connects to the line between an output of the first OTA and an input to the second OTA. When the circuit is lightly loaded, it will have a large, finite voltage gain that is a product of the limiting gains of the OTA's. Neglecting the compensation capacitance, the gain begins to roll off at a frequency determined by load capacitor C.sub.L and by a total load resistance seen by the second OTA, including load resistance R.sub.L and any internal impedance. This result is complicated, by additional poles, the most prominent of which is at the output of the first OTA. This is due to an unavoidable capacitance at the output of the first OTA and an input capacitance of the second OTA. If the two OTAs are similar, the frequencies of the two poles are near each other, thus causing the circuit to have a 40 dB/decade roll-off and marginal stability.
A compensation capacitor C.sub.c may be placed between the output of the first OTA and the output of the circuit to address the uncertainty about load capacitor C.sub.L and its inherent ESR. In the absence of a load capacitor C.sub.L, compensation capacitor C.sub.c may be chosen to give a unity gain frequency lower than a frequency at which other poles affect the response. If load capacitor C.sub.L is large, however, it dominates the response and can roll off the gain before some other pole appears.
Cascaded OTA's each have poles and each requires a stable loop when used in a local feedback loop. This issue becomes a very serious problem in a low drop-out regulator in which an input section and an output device are connected to different supply rails. These regulators have problems that are not easily solved as described for the circuit referred to above.
In one type of positive low drop-out regulator in which an input stage is referred to one supply rail, such as ground, and an output stage is referred to another supply rail, the output stage may include a P-type transistor, such as a PNP or PFET, connected between a supply rail and the load. The P-type transistor causes the regulator to pull the load positive in response to a drive pulling negative on its control electrode. The control signal to the control electrode may be provided by an N-type transistor that receives a control signal from an output of an OTA. This output signal is based on a difference between a reference voltage at a non-inverting input lead and a voltage based on the output signal at an inverting input.